- Use of drugs, within the prior 28 days, contraindicated while taking lopinavir/ritonavir or efavirenz because of effects on the cytochrome P450 system. These include propafenone, astemizole, terfenadine, rifampin, rifapentine, ergot derivatives, cisapride, lovastatin, simvastatin, pimozide, midazolam, and triazolam.